Method for obtaining vaccines for preventing the pathogenic effects related to a retroviral infection

ABSTRACT

The invention provides a method of searching for and obtaining a vaccine against the pathogenic effects related to the infection of an animal or human host by a retrovirus that penetrates into a target cell of the host, and a vaccine obtained by the method are provided. The method includes preparing candidate vaccine agents based on a polypeptide comprising at least part of an envelope protein of a pathogenic strain of the retrovirus and selecting as the vaccine a modified polypeptide chosen from polypeptides that induces an immune response directed against an immunodominant region of an envelope protein of the retrovirus and not against a protein of the host.

The present invention relates to a method for obtaining vaccines for preventing the pathogenic effects related, in humans and in vertebrate animals, to retroviral infections.

The pathogenic effects related to a retroviral infection are the harmful effects, including possible oncogenic or immunosuppressive effects, induced by the introduction of a retrovirus into the body of a host (mammal, bird or alternatively fish), followed by the penetration and by the replication of said retrovirus in the cells of the host which are target cells for the retrovirus, that is to say cells into which the virus is capable of penetrating.

Retroviruses are thus named because they have the capacity, by virtue of the enzyme called reverse transcriptase, of carrying out transcription of RNA to DNA, whereas in living beings, the genetic information usually goes from the DNA of the chromosomes to proteins, via messenger RNA.

Three subfamilies can be distinguished in the retroviral family: the oncoviruses, the lentiviruses and the spumaviruses.

The oncoviruses are retroviruses thus termed because they can be associated with cancers and malignant infections. There may be mentioned, for example, leukemogenic viruses (such as the avian leukemia virus (ALV), the murine leukemia virus (MULV), also called Moloney virus, the feline leukemia virus (FELV), human leukemia viruses such as HTLV1 and HTLV2, the simian leukemia virus or STLV, the bovine leukemia virus or BLV), the primate type D oncoviruses, the type B oncoviruses which are inducers of mammary tumors, or oncoviruses which cause a rapid cancer (such as the Rous sarcoma virus or RSV); see for example STEHELIN et al., J. Mol. Biol. 101 : 349-365 (1976).

The lentiviruses are thus named because they are responsible for slow-progressing pathological conditions which very frequently involve immunosuppressive phenomena, including AIDS.

The appended Table 1 indicates, by way of illustration, the pathological conditions associated with some lentiviruses, as well as the main target cells for these lentiviruses.

The spumaviruses manifest fairly low specificity for a given cell type or a given species, and they are sometimes associated with immunosuppressive phenomena; that is the case, for example, for the simian foamy virus (or SFV).

One of the aims of the present invention is the development of methods and vaccine products intended for effectively preventing the pathogenic effects, including the oncogenic or immunosuppressive effects, related to the infection of a host organism by a retrovirus.

Immunosuppression related to infection has been observed for a large number of retroviruses, and may be considered as a pathogenic constant of retroviral infection; see in particular BENDINELLI et al., Advances in Cancer Research 45 : 125-181 (1985). This is the case in particular for lentivirus infections. It is also the case in a good number of oncovirus infections; see for example P. SONIGO in the book “SIDA et infection par VIH” [AIDS and HIV Infection], MONTAGNIER et al. (Medecine Science Flammarion), pages 113-122 (1989).

Many human and animal vaccines have been tested for preventing the pathogenic effects of retrovirus infections but, as a general rule, these vaccines are not very effective or are ineffective. In particular, in the field of human or animal AIDS, it is observed that, 14 years after the discovery of the HIV virus (BARRE-SINOUSSI et al., Science 220 : 868-871, 1983), it has not yet been possible to find a vaccine which is able to effectively stop a post-vaccine HIV or SIV infection; see for example LINHART et al., AIDS Research and Human Retroviruses 13 : 593-599 (1997); VOGT et al., Vaccine 13 : 202-208 (1995); and LETVIN et al., J. Virol. 69 : 4569-4571 (1995).

The majority of the vaccine preparations used comprise proteins of the retroviral envelope in various forms, for example inactivated viruses, envelope proteins such as the gp 120 and gp 160 proteins of HIV (see in particular GORSE, G. J., Vaccine 10 : 383-388, 1992), virus cores with envelope proteins, or envelope proteins associated with various vectors (chimeric viruses, bacteria); see Levy J. A., Trans. Med. Rev. 2 : 265-271, 1988 and Microbiol. Rev. 57 : 183-289, 1993, in particular page 247.

Other preparations use fragments of the retroviral envelope or immunodominant peptides derived from the envelope glycoproteins, these peptides being presented in various forms (lipopeptides, peptides bound to a supporting protein), so as to make them immunogenic; see in particular Eriksson et al., Vaccine, 11 : 859-865 (1993).

The vaccine strategies conventionally described, for example in the field of human, simian or feline AIDS, recommend not modifying the conserved and immunodominant epitopes of the envelope proteins, which may appear to be completely logical. Indeed, on the one hand, these conserved epitopes are common to different viral strains, which is favorable to the production of a vaccine which has to induce an immune response directed against a majority of strains. On the other hand, these immunodominant epitopes are well recognized by the cellular or humoral immune system during the vaccinal and infectious process and, moreover, they frequently represent neutralization sites; see for example HO et al., J. Virol. 61 : 2024-2028 (1987); JOHNSON et al., J. Exp. Med. 175 : 961-971 (1992); SHAFFERMAN et al., P.N.A.S. U.S.A. 88 : 7126-7130 (1991); and HAMMOND et al., J. Immunol. 146 : 1470-1477 (1991).

The method of the invention consists, by contrast, in modifying the conserved and immunodominant epitopes of certain proteins of the viral envelope, in order to obtain an effective vaccine.

Indeed, the authors of the present invention have discovered that conserved and immunodominant regions of the retroviral envelope may be responsible for harmful autoimmune phenomena. By way of example, in the case of human AIDS, they have observed that certain conserved and immunodominant regions of the HIV envelope exhibit three-dimensional structural analogies and/or cross-reactions with certain regions of at least one protein of the human immune system, such that the administration, as a vaccine, of a viral protein containing said intact regions induces an immune response which is responsible for harmful autoimmune reactions leading to vaccine failure.

At the origin of the present invention, there is, on the one hand, the observation mentioned above that conserved and immunodominant regions of certain retroviruses, usually present in vaccine preparations, are precisely, in a good number of cases, regions which cause harmful autoimmune reactions because they exhibit three-dimensional structural analogies and/or cross-reactions with certain proteins of the host for the virus. At the origin of the present invention, there is also, on the other hand, the observation that said proteins of the host use the same target cell, or the same target cells, as said retroviruses. All these observations carried out by the authors of the invention have led them to think that the retroviral envelope proteins and the host proteins which exhibit three-dimensional structural analogies and/or cross-reactions bind in many cases to the same target cells and possess, on these target cells, common membrane receptors.

It is said that a protein exhibits cross-reactivity with another protein when it is possible to obtain, by in vivo or in vitro immunization with the aid of one of said proteins, an immune response also directed against the other protein, for example when this immunization induces a (so-called B type) humoral response and makes it possible to obtain and to select at least one monoclonal antibody which is capable of recognizing the other protein, or when the same cellular immune response (that is to say of the T type) induced in vitro by one of the proteins recognizes the two proteins, according to the known tests for detecting a T-type immune response, such as for example the tests for cytotoxicity in vitro. It is known that the term “immunization” denotes the process of induction of an immune response following stimulation, by bringing immunocompetent cells of a host into contact in vivo or in vitro with an antigen, and that one of the aims of the administration of a vaccine agent is precisely to obtain such an immunization.

The subject of the invention is therefore a method of obtaining a vaccine against the pathogenic effects related to the infection of an animal or human host by a retrovirus capable of penetrating into a target cell of said host, said target cell possessing a membrane receptor for a protein of said host, method in which a vaccine agent based on a polypeptide comprising at least part of an envelope protein of a pathogenic strain of said retrovirus is prepared, and in which said polypeptide is prepared in a modified form, it being understood that:

-   -   said part of the envelope protein is chosen from those which         comprise at least one fragment of an immunodominant region of         said envelope protein, said fragment containing at least one         amino acid which is a conserved amino acid of said         immunodominant region and which is present in said pathogenic         strain,     -   said polypeptide, in the unmodified state, induces an immune         response directed both against said immunodominant region and         against the protein of the host,     -   and said modified polypeptide is chosen by [sic] those which         induce an immune response directed against said immunodominant         region of the envelope protein and not against the protein of         the host.

In the definition of the method of the invention which has just been given, the vaccine agent is said to be “based” on a modified polypeptide. This means that the vaccine agent comprises such a modified polypeptide, but this does not mean that the vaccine agent is necessarily of an exclusively polypeptide nature. In fact, in this vaccine agent, said polypeptide may be optionally bound to (in particular covalently) or associated, in a manner known per se, with any biocompatible molecule which may be chosen, for example from polymers, lipids, peptides (including lipopeptides, glycopeptides, proteins), nucleic acids, oligosaccharides and the like. Said biocompatible molecule may in particular serve as a support for the polypeptide immunogenic agent. It can also serve to modify the conformation of the polypeptide and, in the latter case, said molecule should be considered as a substituent modifying the amino acid residue to which it is attached, said substituent thus modifying, in the final analysis, the antigenicity of the polypeptide of which this amino acid residue is a part.

The method of the invention may comprise, at least in a preliminary research phase, a step consisting in selecting the polypeptides (unmodified) comprising at least part, as defined above, of the viral envelope protein of a pathogenic strain of the retrovirus. This protein part, which comprises at least one immunogenic fragment of an immunodominant region, is such that the polypeptide (unmodified) is capable of inducing an immune response directed both against the viral protein (more precisely against the fragment of the immunodominant region contained in said part) and against the protein of the host, and it is the existence of such an immune response, directed against the viral envelope protein and against the protein of the host, which defines, in the present application, the pathogenic character of a viral strain. It is thus possible to select the polypeptides (unmodified) comprising such a fragment.

A polypeptide fragment is said to be immunogenic if the immunization of a host, in vivo or in vitro, with said fragment, optionally bound to an appropriate support (such as a protein, a lipid or a polypeptide), makes it possible to obtain an immune response (of the B type and/or of the T type, directed against said polypeptide fragment).

In the present application, when reference is made to an immune response, without any other specific information, it is an immune response of a vertebrate, following immunization in vitro or in vivo.

The method of the invention may also comprise at least one step consisting in modifying, in the manner which will be indicated below, a polypeptide thus selected, and in choosing among the polypeptides those modified, at least one modified polypeptide which induces an immune response directed against the viral envelope protein and not against the protein of the host.

Thus, while the prior art taught, as noted above, not to modify the conserved and immunodominant epitopes of the retroviral envelope proteins, the aim of the method of the invention is, by contrast, to modify the antigenicity of such epitopes so as to obtain a differential immune response with respect to the viral envelope protein and to a protein of the host.

It is known that in order to modify the antigenicity of an immunogenic fragment of a polypeptide, it is possible to modify said polypeptide with the aid of a mutation affecting at least one amino acid. A definition will be given later of what “mutation” should be understood to mean here. The mutated amino acid may be present in the immunogenic fragment, or even in a region of the polypeptide outside said fragment. It is in fact known that the modification of an amino acid situated outside a fragment can affect the spatial structure of said fragment and therefore its antigenicity; in particular, it has been shown that the conformation of an amino acid residue, in a peptide, can be influenced by the nature of the amino acid residues at positions going from +8 to −8 relative to this amino acid residue; see for example GARNIER et al., J. Mol. Biol. 120 : 97-120 (1978). Beyond this, the nature of the amino acid residues still has an influence, but this influence is neither symmetrical nor quantifiable from the sole knowledge of the peptide sequence considered.

A mutated amino acid can therefore be situated in the modified polypeptide, inside or outside the immunogenic fragment. When it is outside the immunogenic fragment, it is generally not separated from the nearest end of said immunogenic fragment, in the polypeptide chain, by more than eight (and in particular by more than seven) amino acid residues. In particular, an amino acid, mutated in accordance with the present invention, and situated outside the immunogenic fragment, is generally not separated by more than eight amino acid residues, and in particular by more than seven amino acid residues, from the nearest conserved amino acid belonging to the immunodominant region of which at least one fragment is contained in the unmodified polypeptide.

The modified polypeptide in accordance with the present invention may be, for example, the whole envelope protein of a pathogenic viral strain, modified by at least one mutation as indicated above. The modified polypeptide may also be part of the envelope protein of a pathogenic viral strain, modified by at least one mutation as indicated above, said part comprising at least one immunogenic fragment as defined above. The modified polypeptide may also be a chimeric protein comprising at least part of the envelope protein, said part of the envelope protein being as defined above and comprising at least one mutation.

The modified polypeptide used according to the invention may be, for example, a transmembrane glycoprotein of a retrovirus or a fragment of a transmembrane glycoprotein, in particular a fragment comprising an outer region of said transmembrane glycoprotein (modified), that is to say a region which is present on the outer surface of the viral membrane. Of course, such a protein fragment comprises at least part of an immunodominant region, as indicated above. When reference is made to an “outer” region of a protein, it is more precisely its surface which is accessible to the solvent, which may be defined in particular with the aid of software such as X-plor (see below) using the algorithm described by Lee & Richards, J. Mol. Biol. 55 : 379-400, 1971. Said polypeptide may also be in the form of an oligomer of at least part of said transmembrane glycoprotein, in the modified state.

The definition given above of the method of the invention implies that the polypeptide used comprises at least part of an immunodominant and conserved region of a viral envelope protein. In the description of the present application, “conserved region” describes a region, optionally reduced to a single amino acid residue, of the viral protein, where, for a majority of strains of a given virus (for example in at least 6 strains out of 10 approximately), there are one or more identical or functionally analogous amino acids situated at the same position in peptide sequence alignments of said protein of the various strains. Such an identical or functionally analogous amino acid is called conserved amino acid. The notion of conservation of functionally analogous amino acids is known, and there are numerous substitution matrices which make it possible to quantify this notion (Dayhoff, M. O. et al., in Atlas of Protein Sequence and Structure, 1978, Vol. 5, Suppl. 3, Chapters 22 and 23).

The conserved regions can be easily determined, after sequencing of proteins of various strains of the virus studied, by methods of multiple alignments of the sequences obtained. For that, it is possible to use, for example, the Clustal-w program (Thompson J. D. et al., Nucleic Acids Research 22 : 4673-4680, 1994). Moreover, the protein sequences of various viral strains are often accessible on data banks. For example, the Web server of the Los Alamos HIV data base has the HIV1, HIV2, SIV and FIV sequences which are regularly updated. The address of this server on the Internet network is:

http://hiv-web.lanl.gov/HTML/sequences.html

The appended Tables 2a, 2b, 2c and 2d are examples of sequence alignment of the regions belonging to the homologous envelope glycoproteins of region 545-682 of the HIV transmembrane glycoprotein (entry SWISSPROT ENV_HV1 BR), respectively for HIV1, HIV2, FIV and SIV. The last line of the tables summarizes, with the aid of symbols, the degree of homology and therefore the degree of conservation observed. The symbol “*” indicates a position of the alignment where the same residue is present in all the sequences, the symbol “:” indicates a position in the alignment where the amino acids present in the various sequences are very similar, the symbol “.” indicates a position in the alignment where the amino acids present in the various sequences are similar, and the absence of a symbol indicates a position in the alignment where the amino acids present in the various sequences are not very similar. This symbolic system is used by the Clustal W alignment program (version 1.7).

In the description of the present application, immunodominant region of a protein refers to a peptide sequence which induces, in a great majority of cases (for example in at least 7 cases out of 10 approximately), a humoral and/or cellular response of the immune system directed against said region after immunization with a protein containing said sequence or with a peptide essentially consisting of said sequence.

The definition of the method of the invention makes reference to the target cells of a virus which are the cells into which the virus is capable of penetrating. The target cells of retroviruses are generally known. Viruses have the property of binding to the cells which they are capable of infecting. It is therefore optionally possible to test for, using routine experiments in vitro, the target cells of a virus studied.

The definition of the method of the invention also makes reference to the cells of the host having a membrane receptor for a protein of the host. The cells of the host which have a receptor for a protein of said host are often known and, in the opposite case, it is possible, using routine experiments, to determine if a given protein binds to a certain type of cell. It is possible, for example, to use a radiolabelled protein and to determine if it binds to said cell type. It is also possible to test if the protein binds to a given membrane receptor using a cell line transfected with a gene expressing said membrane receptor.

The proteins of the host for which certain cells of the host possess a membrane receptor are mainly proteins belonging to the range of soluble protein mediators. This range includes proteins called, depending on the cases, hormones, growth factors or cytokines, although there is no distinct boundary between these various categories of mediators; see for example CAVAILLON J. M., Les Cytokines (Masson, Paris, 1996) Chapter 1, pages 1-3 and preface.

In the present application, it is considered that an immune response, for example an antibody response, obtained by immunization with the aid of the modified polypeptide prepared in accordance with the method of the invention, is directed against the viral envelope protein and not against the protein of the host, when the antibodies obtained have affinities for the protein of the host and for the envelope protein of the retrovirus which exhibit a substantial difference, resulting in particular in differences in reactivity which are considered to be very significant in ELISA tests, such as for example optical densities in a ratio of about 4 (or more), which means that the optical density observed after attachment of said antibodies to the viral protein is at least four times higher than that observed for the attachment of said antibodies to the protein of the host. Similarly, a cellular type immune response is considered to be directed against the envelope protein but not against the protein of the host when the immunization in vitro of immunocompetent cells of the host with the candidate vaccine induces the formation of activated cells whose reaction toward cells (including transfected cell lines) expressing the retroviral envelope protein is significantly higher than the reaction toward cells expressing the protein of the host, for example when, in the final optical measurement, or in the final radioactivity counting (in particular ⁵¹Cr radioactivity released by target cells) of the test used, or alternatively in the assessment by any known means of a cell lysis caused by induced cytotoxic cells, the scales of response are in a ratio of about 4 (or more). The criteria which have just been indicated make it possible at least to make a first choice among the modified peptides studied, but in the final analysis, it is the absence or the decrease in the pathogenic effect due to the suppression or the weakening (demonstrated by any appropriate means) of the immune response toward the protein of the host which will constitute the criterion for selection of the modified peptides capable of constituting satisfactory vaccine agents.

The immunodominant and conserved regions of which it is desired to modify the antigenicity, in accordance with the invention, may be chosen from those which give in vitro a cross-reaction, of the B type and/or of the T type, with the host protein defined above.

It is also possible to choose such an immunodominant and conserved region from those for which a three-dimensional structural analogy with a region of said protein of the host has been determined beforehand, said structural analogy being capable of being associated with a cross-reaction in vitro and/or in vivo. The three-dimensional structural analogy between certain regions of two proteins refers to equivalent arrangements, in space, of amino acid residues which are similar because, in particular, of their side chain and/or of their analogous functional chemical groups. The three-dimensional structures of the proteins can be obtained with the aid of nuclear magnetic resonance (NMR) spectra and/or of X-ray diffraction spectra. For example, the structure of the SIV gp41 protein was obtained with the aid of the NMR spectrum (Caffrey M. et al., J. Mol. Biol. 271, 819-826, 1997). In addition, it is possible, in some cases, to obtain a good model with the aid of molecular modeling techniques, from the atomic coordinates of a protein of known structure. It is possible to use for that, in particular, the molecular modeling software X-plor (reference: “A system for X-ray crystallography and NMR, Version 3.1”, Axel T. Brunger, Yale University Press, 1992).

To search for a three-dimensional structural analogy, it is possible to use, for example, the known methods of visualization and superposition on a graphic screen of the three-dimensional structure of biological molecules. Software exists which allows the visualization of the three-dimensional structures of the molecules with different modes of representation, the calculation of the geometric parameters (such as distances, angles and the like) and the objective and quantitative superposition of several molecular structures (in particular RASMOL software: Sayle, R. A. and Milner-White E. J., J. Mol. Biol., 247, 536-540, 1995 and ANTHEPROT software: Geourjon C. and Deléage G., J. Mol. Graph. 13, 209-212, 1995) as well as the estimation of the accessibility to solvents (X-plor software, already mentioned, and CCP4 software: Collaborative Computational Project Number 4, Acta Cryst., D50, 760-763, 1994.

However, in order to have a finer estimation of these structural analogies, it is useful to consider, at the level of each amino acid, the functional groups positioned in a similar manner in both proteins which are compared. For that, the co-inventors of the present invention use methods which make it possible to calculate molecular surface areas with the aim of comparing functional properties between two three-dimensional structures, in order to take into account, not amino acids in their entirety, but also, more particularly, functional chemical groups of each of them (for example: amide, carboxyl, hydroxyl, sulfhydryl and amine functions and the like). It is thus possible to take into consideration, in the structures compared, functionally analogous amino acids, and not only identical amino acids.

It is therefore considered that a region of a retroviral protein exhibits a three-dimensional structural analogy with a given region of a protein of the host when the techniques which have just been mentioned make it possible to demonstrate, in the two regions compared, a similar spatial organization of certain identical or functionally analogous amino acids.

It should be noted that amino acids which are functionally analogous and grouped together in a similar manner in space can be relatively distant from each other in the same peptide chain. However, the three-dimensional structural analogy between two proteins which are being compared can also relate to the spatial arrangement, in a similar manner, of identical or functionally analogous amino acids in the case where, one of the proteins being oligomerized, the amino acid residues involved are situated on different chains of the oligomer, whereas the amino acid residues of the other protein which are involved in this analogy can be situated on the same peptide chain of this other protein.

It is particularly advisable to search for three-dimensional structural analogies and/or cross-reactions with regions of the protein of the host which are involved in the attachment of said protein to its receptor.

Among the proteins of the host which are mentioned in the definition of the method of the invention, there may be mentioned in particular the soluble mediators as defined above. Taking into account the remark made above that immunosuppressive effects are generally associated with retroviral infections, it is particularly important to search for structural analogies and/or for cross-reactions between an outer protein of a retrovirus and soluble protein mediators of the immune system. Among these immune system mediators, there may be mentioned cytokines, and in particular interleukin-2, interleukin-10, interleukin-15 as well as interleukin-8 and chemokines.

To prepare the modified polypeptide which constitutes the vaccine agent obtained according to the invention, it is possible to use known methods of peptide synthesis or genetic engineering techniques. It is possible to isolate or to prepare a polynucleotide sequence encoding at least part of the envelope protein of the virus and, if desired, it is possible to introduce at this stage, into the nucleotide sequence, mutations which make it possible to obtain a mutated product of translation which constitutes the modified polypeptide. It is also possible to directly synthesize a modified polynucleotide sequence comprising one or more mutations and encoding the modified polypeptide. The mutated polynucleotide sequences thus obtained are introduced in a known manner into an appropriate vector which makes it possible to express said polypeptide, optionally in modified form. Such a vector is for example E. coli, a baculovirus or a mammalian cell. It is also possible to carry out the mutation on an unmodified polypeptide obtained according to one of the preceding methods.

In the present application, “mutation” refers to any modification of a region (optionally reduced to a single amino acid residue) of a polypeptide, by physical means, chemical means (covalent or noncovalent modification) and/or biological means (mutations by substitution, deletion and/or insertion of one or more amino acids), leading to the modification of the functional potentials of the constituent amino acid(s) of said region, termed “mutated region”. By way of example, it is possible to carry out mutations leading to the abolition, acquisition and/or modulation of the properties of disulfide bridges, hydrogen bonds, electrostatic interactions and/or hydrophobic interactions, the modification of the capacity of a protein to form a heterocomplex, or alternatively, in the case of an oligomeric protein, the modification of the state of oligomerization or of the stability of the oligomer.

The modification of an amino acid a of a polypeptide chain (including the modification of a terminal amino acid of the polypeptide considered) can influence the conformation of the neighboring amino acids in the chain, including, as was recalled above, the conformation of an amino acid b separated from a by a number of amino acid residues which may be as high as seven or eight, and when the amino acid b is part of an epitope, any modification of the amino acid a (in particular any addition of a substituent or any modification of a substituent) is capable of modifying the antigenicity of the epitope considered.

In the phase for searching for modified polypeptides in accordance with the invention, the choice of the amino acids to be mutated and/or the choice of the mutation methods can be made in an arbitrary manner or in a reasoned manner. It is possible to use in particular at least one of the following methods of modification:

-   -   1) the replacement of one or more amino acids having a         hydrophobic side chain (examples: Ala, Leu, Val, Ile, Phe, Trp,         Met, Tyr, Cys) by one or more amino acids having a hydrophilic         side chain (examples: Arg, Asp, Asn, Glu, Gln, Ser, Thr, His,         Lys) or an indifferent chain (examples: Gly, Pro) and vice         versa;     -   2) the replacement of one or more amino acids having a         positively charged side chain (examples: Arg, Lys, His) by one         or more amino acids having a negatively charged side chain         (examples: Asp, Glu) or a neutral chain and vice versa;     -   3) the acquisition, suppression and/or modification of one or         more disulfide bridges;     -   4) the production of mimotopes, in particular which are obtained         by retro-inversion;     -   5) the substitutions, suppressions, additions and/or other         modifications of at least one amino acid which is potentially a         donor or acceptor of hydrogen bonds;     -   6) the substitutions, suppressions, additions and/or other         modifications of at least one amino acid which is potentially a         donor or acceptor of ionic bonds;     -   7) the change in steric hindrance by substitutions,         suppressions, additions and/or other modifications of one or         more amino acids;     -   8) the use of amino acids which are not naturally present in         proteins;     -   9) the modification of glycosylation (creation, suppression or         modification of glycosylation sites or of their associated         sugars).

Of course, the modified polypeptides thus obtained are tested, as indicated above, in order to select the modified polypeptides which induce an immune response directed against the envelope protein and not against the protein of the host.

The method of the invention may be applied to the obtaining of vaccines, in particular against the following viruses: HIV, FIV, SIV, leukomogenic oncoviruses (avian, murine, feline, human, simian and bovine leukemia viruses, that is to say, respectively, ALV, MULV, FELV, HTLV, STLV, BLV), primate type D retroviruses, mammary tumor-inducing type B retroviruses, Rous sarcoma virus, maedi-visna virus (infecting sheep), feline sarcoma virus, avian myelocytomatosis virus and avian myeloblastosis virus.

The subject of the invention is also the use of a modified polypeptide, as defined above, in the preparation of a vaccine composition for preventing the pathogenic effects related to the infection of a host by a retrovirus.

The invention also relates to a vaccine composition which can be obtained by the method of the invention, and containing as active ingredient a modified polypeptide as described above. Such a composition may be used in a method of vaccination for preventing the pathogenic effects of retrovirus infections, this method essentially consisting in administering to a vertebrate animal, including a human, a modified polypeptide as defined above in a sufficient quantity to obtain a vaccination effect. The formulation of the vaccine compositions, and their method of administration, are known per se and will not be further described here.

The subject of the invention is also a modified retroviral polynucleotide encoding a modified polypeptide as defined above. The modified polynucleotide may be obtained as indicated above. The invention extends to an expression vector into which said modified polynucleotide has been inserted, said expression vector being thus capable of expressing the modified polypeptide.

The modified polypeptide obtained according to the invention can also serve as immunogenic agent in order to induce, by immunization, the formation of antibodies which can be used in particular in the treatment of retroviral infections, and the invention therefore also relates to the antibodies obtained in response to the immunization of animals (including humans), in vivo or in vitro, with the aid of the vaccine agent containing a modified polypeptide described above. The antibodies of the invention are in particular purified polyclonal antibodies or monoclonal antibodies exhibiting the characteristic of recognizing the retroviral envelope protein without recognizing the protein of the host. The purification of the polyclonal antibodies, and the selection of the monoclonal antibodies, can be carried out with the aid of the viral protein and of the protein of the host, so as to select only the antibodies which recognize the viral protein and not the protein of the host. The antibodies of the invention can be used in particular in the early treatment of infections caused in said host by the retrovirus against which they are directed. The dosage is the usual dosage for antibodies. The pharmaceutical compositions containing such antibodies also constitute one of the subjects of the invention.

The following examples illustrate the invention.

EXAMPLE 1

The manner in which the authors of the present invention searched for structural and antigenic analogies between an envelope protein of a virus, namely the gp41 protein of HIV1, and a cytokine, namely human interleukin-2 (abbreviated: IL-2), is presented below.

It will be noted that the peptide sequences of gp41 and of human IL-2 are not homologous. Indeed, the two proteins exhibit overall between them only 16.5% sequence identity, this threshold of homology not being significant, as can be easily noted with the aid of in silico simulations carried out on sequences having the same composition but in which the order of the amino acids has been randomly modified.

Previous studies (Bost et al., Immunology 65: 611-615, 1988) had reported a sequence homology between the protein gp41 and IL-2 (sequence LERILL). It should be noted that this LERILL sequence of gp41 does not constitute an immunodominant region of this protein; see LEVY J. A., Microbiol. Rev. 57: 183-289 (1993) in particular page 232. The sequence LERILL is in fact situated inside the viral particle; it corresponds to the C-terminal part of gp41.

Having observed that IL-2 and the AIDS-related retroviruses appear to have common target cells, the authors of the present invention made the hypothesis that the receptor for human interleukin-2 could be common to IL-2 and to the gp41 protein of HIV, and they therefore searched for possible three-dimensional structural analogies between the latter two.

In the present application, the numberings of the amino acid residues of the peptide sequence of interleukin-2 and of gp41 are those used in the SWISSPROT bank (version 34).

The peptide sequences of IL-2 and of the gp41 protein are known. In the present application, reference is made to the following published sequences:

-   -   for IL-2: SWISSPROT entry (version 34) which has the code         IL2_HUMAN;     -   for gp41: SWISSPROT entry (version 34) which has the code         ENV_HV1BR.

The published structures which have been used are the following:

-   -   for IL-2: PDB entry (Brookhaven Databank) 11RL;     -   for gp41: PDB entries (Brookhaven Databank)         -   1AIK,         -   1ENV.

Moreover, the three-dimensional structure of IL-2, determined by NMR, is known (Mott, P. C. et al., J. Mol. Biol., 248: 979, 1995), as well as the structure of certain domains of the gp41 protein, which was obtained with the aid of the X-ray diffraction spectrum (Chan, D. C. et al., Cell, 89, 263-273, 1997; Weissenhorn, W. et al., Nature, 387, 426-430, 1997). Moreover, a three-dimensional model of part of the outer domain, in the 545-671 region, of the gp41 protein (trimeric form) was obtained, by molecular modeling, by the coinventors of the present invention. This molecular model was obtained using the X-plor software by a strategy similar to that of molecular modeling under NMR constraints. The constraints necessary for molecular modeling of the trimeric form were deduced from the three-dimensional structure of the pII mutant of the “leucine zipper” domain of the protein GCN4 (PDB code: 1GCM), crystallized in the form of a trimer of the “coiled coil” type.

By examining the structures obtained, three-dimensional analogies were found between certain regions of the gp41 protein and certain regions of interleukin-2 participating in the attachment to its receptor. The mode of attachment of IL-2 to its receptor, as well as the regions of IL-2 involved in this attachment, are indeed known; see BAZAN J. F., P.N.A.S. USA 87: 6934-6938 (1990); Bamborough P. et al., Structure 2: 839-851 (1994); Gnarra J., R. et al., P.N.A.S. USA 87: 3440-3444 (1990); Takeshita T. et al., Science 257: 379-382 (1992); and CAVAILLON J. M., Les Cytokines (Masson, Paris, 1996), pages 119-125.

These results have been confirmed by studies of overall comparison of the structures of gp41 and of IL-2, and also by local comparisons made by focusing more particularly on the analogous functional groups of each of the structures, as already indicated in the description above.

It was observed, in particular, that regions 53-61 and 88-93 of IL-2, organized in alpha-helix form, are superposed in a satisfactory manner with two of the three helices of the central trimer of gp41. This implies that in the two proteins, groups carried by different helices can have comparable properties of accessibility and relative organization.

Local three-dimensional structural analogies were also found between a highly conserved immunodominant region of the gp41 glycoprotein of HIV (more precisely in region 545-682) and human interleukin-2.

The peptide sequence of region 545-682 of the gp41 protein of HIV1 (SWISSPROT code: ENV_HV1BR) is reproduced in the appended Table 3.

In the appended Table 3bis, the peptide sequences of four regions of this region of gp41 (555-577, 572-601, 590-620 and 628-663) have been represented, in which structural analogies and/or cross-reactions were noted with IL-2.

The regions of IL-2 concerned by the structural analogies which have just been mentioned are the regions 27-47, 45-69, 99-121 and 131-153 of IL-2. The peptide sequences of these regions are represented in the appended Table 4.

It is important to note that region 27-47 of IL-2 is involved in the attachment of IL-2 to the beta chain of its receptor. Indeed, the amino acids in region 27-47 belong to the A helix which participates in the attachment to the receptor for IL-2 (RIL-2), more precisely to beta RIL-2.

The amino acids in region 45-69 belong to a region of IL-2 which participates in the attachment to alpha RIL-2.

The amino acids in region 99-121 belong to the E helix participating in the attachment to beta RIL-2.

The amino acids in region 131-153 of IL-2 belong to the F helix participating in the attachment to gamma RIL-2.

By way of illustration, the structural analogies which were found between region 572-601 of gp41 and region 27-47 of human IL-2 are specified in the appended Table 4bis. The outer amino acids involved in this three-dimensional structural analogy are underlined in Table 4bis.

It should be noted, however, that the same region of gp41 can exhibit three-dimensional structural analogies with several distinct regions of IL-2.

In addition, the authors of the invention have observed that in region 600-612 of gp41, the three lysines (K) at position 606 on the three chains of gp41 trimer are capable of forming a conformational epitope, it being possible for these lysines of gp41 to correspond, in space, to lysines 52, 96 and 55 of IL-2.

They also observed immunological cross-reactivities between the IL-2 and gp41 proteins. In particular, using the ELISA and PEPSCAN techniques, with antibodies obtained from HIV+ sera purified by immunopurification on a column containing immobilized human IL-2, they observed that some of these antibodies recognize regions of IL-2 involved in the attachment of IL-2 to the alpha, beta and gamma chains of its receptor, and in particular regions belonging to the A helix (KTQLQLEHLLLTLQ), the E helix (RPRDLISNINVIVLELK), the F helix (TIVEFLNRWITFCQSIISTLT), the AB loop and the beginning of the B helix (NNYKNPKLTRMLTFKFYMPKK).

Using the filter dot blot techniques and the Western blot-type immunotransfer techniques, it was also shown that the polyclonal antibodies obtained from sera of HIV+ patients, and immunopurified on human IL-2, recognize oligomers of the gp41 protein.

The studies carried out also showed that murine and human anti-gp41 monoclonal antibodies directed against immunodominant conserved regions of the gp41 protein of HIV recognize regions of IL-2 which participate in the attachment of the latter to the alpha, beta and gamma chains of the receptor for IL-2.

It is therefore possible to obtain vaccines against the HIV virus, in accordance with the invention, in particular by preparing polypeptides containing at least one of the regions of gp41 described in Table 3bis, said polypeptides being in modified form, that is to say containing at least one mutation, as indicated above. It should be clearly understood that these divisions of the region 545-682 into regions can have a certain arbitrary character, and that is why some regions indicated may overlap.

EXAMPLE 2 Mutations on gp41 of HIV1

Mutated gp41 envelope glycoproteins are prepared according to known methods. These mutations are described in the appended Table 5 which represents the relevant sequence of gp41 and, aligned under the latter, the mutated sequences. The level of the mutations is indicated by underlining the relevant amino acids.

Vaccine compositions are prepared each comprising, in sterile and pyrogen-free, aqueous saline solution, one of the mutated gp41 proteins obtained above.

Rabbits or mice are immunized with the mutated proteins obtained and it is determined whether the antibodies developed by these animals recognize or do not recognize human interleukin-2, for example by the ELISA or PEPSCAN technique. The mutated proteins which induce the formation of antibodies not recognizing IL-2, but recognizing the gp41 protein, are selected.

The PEPSCAN technique is described by J. WORTHINGTON and K. MORGAN, “Epitope mapping using synthetic peptides”, in “PEPTIDE ANTIGENS-A practical approach” (G. B. WISDOW Ed.), Oxford University Press (1994).

EXAMPLE 3 Mutations on gp36 of FIV

The sequence of the gp36 protein of FIV is known (reference ENV_FIVPE).

Some of the sequences of this protein, homologous to the conserved regions of gp41 which are described in Table 3bis, have been represented in the appended Table 6, with examples of mutations. TABLE 1 PRINCIPAL TARGET LENTIVIRUS HOST CELL EIAV Horse Macrophage Hemolytic anemia VISNA VIRUS Sheep Macrophage Maedi-visna: encephalitis- interstitial pneumonia CAEV Goat Macrophage Immunodeficiency- encephalopathy- arthritis BIV Bovine T lymphocyte Immunodeficiency- bovine lymphocytosis FIV Cats + Felidae T lymphocyte Immunodeficiency (AIDS) SIV Primates T lymphocyte Immunodeficiency (monkeys) (AIDS) HIV Humans T lymphocyte Immunodeficiency (AIDS) EIAV denotes the equine infectious anemia virus CAEV denotes the caprine encephalitis virus FIV means: feline immunodeficiency virus SIV means: simian immunodeficiency virus HIV means: human immunodeficiency virus

TABLE 2a GP41_HV1Z2 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1Z6 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HVIEL QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1ND QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1MA QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1Z8 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1C4 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1S1 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1BN QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1JR QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1J3 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1SC QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1KE QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1Y2 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS CP41_HV1MN QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS C241_RV1A2 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1OY QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1RH QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1S3 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS CP41_HV1H2 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1H3 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1B1 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1PV QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1B8 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1MF QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1BR QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1W1 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS GP41_HV1W2 QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS CP41_HV1ZR QARQLLSGIVQQQNNLLPAIEAQQHLLQLTVWGIKQLQARILAVERYLKDQQLLGIWGCS *** *:.***:****** **..***:*:************:**** **:**:*:*:**** GP41_HV1Z2 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1Z6 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1EL GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_RV1ND GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1MA GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1Z8 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1C4 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1S1 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1BN GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_Hv1JR GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1J3 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1SC GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1KB GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1Y2 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1MN GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1A2 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1OY GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1RH GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HVlS3 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1H2 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HVIH3 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1B1 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1PV GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1BB GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1MF GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1BR GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1W1 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1W2 GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE GP41_HV1ZH GKLICTTTVPWNSSWSNRSLNDIWQNMTWMEWEREIDNYTGLIYRLIEESQTQQEKNEQE *: :* * ****:****:: . :* * **::*::*::*** *: *::.** ** *::: GP41_HV1Z2 LLELDKWASLWNWFNITQ GP41_HV126 LLELDKWASLWNWFNITQ GP41_HV1EL LLELDKWASLWNWFSITQ GP41_HV1ND LLELDKWASLWNWFSITK GP41_HV1MA LLELDKWASLWNWFSISK GP41_HVLZ8 LLQLDKWASLWNWFSITK GP41_HV1C4 LLQLDKWASLWTWSDITK GP41_HV1S1 LLELDKWASLWNWEDISK GP4I_HV1BN LLELDKWASLWNWFNITN GP41_HV1JR LLELDKWASLWNWFGITK GP41_HV1J3 LLGLDKWASLWNWFTITN GP41_HV1SC LLELDKWASLWNWFNITN GP41_HV1KB LLALDKWDSLWNWFSITK GP41_HV1Y2 LLALDKWASLWNWFDITK GP41_HV1MN LLELDKWASLWNWFDITN GP41_HV1A2 LLELDKWASLWNWFSITN GP41_HV1OY LLELDKWAGLWSWFSITN GP41_HV1RH LLELDKWANLWNWFDITQ GP41_HV1S3 LLELDKWASLWNWFSITN GP41_HV1H2 LLELDKWASLWNWFNITN GP41_HV1H3 LLELDKWASLWNWFNITN GP41_HV1B1 LLELDKWASLWNWFNITN GP41_HV1PV LLELDKWANLWNWLNITN GP41_HV1B8 LLELDKWASLWNWFNITN GP41_HV1MF LLELDKWASLWNWFNITN GP41_HV1BR LLELDKWASLWNWFNITN GP41_HV1W1 LLELDKWASLWNWFSITN GP41_HV1W2 LLELDKWASLWNWFDITN GP41_HV1ZH LLALDKWANLWNWFDISN ** **** ** * *::

TABLE 2b GP41_HV2D1 QSRTLLAGIVQQQQQLLDVVKRQQEMLRLTVWGTKNLQARVTAIEKYLKDOAOLNSWGCA GP41_HV2C1 QSRTLLAGIVQQQQQLLDVVKRQQEMLRLTVWGTKNLQARVTAIEKYLKDOAOLNSWGCA GP41_HV2BE QSRTLLAGIVQQQQQLLDVVKRQQEMLRLTVWGTKNLQARVTAIEKYLKDOAOLNSWGCA GP41_HV2NZ QSRTLLAGIVQQQQQLLDVVKRQQEMLRLTVWGTKNLQARVTAIEKYLKDOAOLNSWGCA GP41_HV2CA QSRTLLAGIVQQQQQLLDVVKRQQEMLRLTVWGTKNLQARVTAIEKYLKDOAOLNSWGCA GP41_HV2RO QSRTLLAGIVQQQQQLLDVVKRQQEMLRLTVWGTKNLQARVTAIEKYLKDOAOLNSWGCA GP41_HV2S2 QSRTLLAGIVQQQQQLLDVVKRQQEMLRLTVWGTKNLQARVTAIEKYLKDOAOLNSWGCA GP41_HV2ST QSRTLLAGIVQQQQQLLDVVKRQQEMLRLTVWGTKNLQARVTAIEKYLKDOAOLNSWGCA GP41_HV2SB QSRTLLAGIVQQQQQLLDVVKRQQEMLRLTVWGTKNLQARVTAIEKYLKDOAOLNSWGCA GP41_HV2D2 QSRTLLAGIVQQQQQLLDVVKRQQEMLRLTVWGTKNLQARVTAIEKYLKDOAOLNSWGCA **** : ******** :********:********* ************ .**:******* GP41_HV2D2 FRQVCHTTVPWVNDSLTPDWNNHTWOEWEKRVHYLEANISQSLEQAOIQQEKNMYELQKL GP41_HV2G1 FRQVCHTTVPWVNDSLTPDWNNHTWOEWEKRVHYLEANISQSLEQAOIQQEKNMYELQKL GP41_HV2BE FRQVCHTTVPWVNDSLTPDWNNHTWOEWEKRVHYLEANISQSLEQAOIQQEKNMYELQKL GP41_HV2NZ FRQVCHTTVPWVNDSLTPDWNNHTWOEWEKRVHYLEANISQSLEQAOIQQEKNMYELQKL GP41_HV2CA FRQVCHTTVPWVNDSLTPDWNNHTWOEWEKRVHYLEANISQSLEQAOIQQEKNMYELQKL GP41_HV2RO FRQVCHTTVPWVNDSLTPDWNNHTWOEWEKRVHYLEANISQSLEQAOIQQEKNMYELQKL GP41_HV2S2 FRQVCHTTVPWVNDSLTPDWNNHTWOEWEKRVHYLEANISQSLEQAOIQQEKNMYELQKL GP41_HV2ST FRQVCHTTVPWVNDSLTPDWNNHTWOEWEKRVHYLEANISQSLEQAOIQQEKNMYELQKL GP41_HV2SB FRQVCHTTVPWVNDSLTPDWNNHTWOEWEKRVHYLEANISQSLEQAOIQQEKNMYELQKL GP41_HV2D2 FRQVCHTTVPWVNDSLTPDWNNHTWOEWEKRVHYLEANISQSLEQAOIQQEKNMYELQKL *******:*** *::*:*:*.*:***:**:::: *:***: **:**:***********: GP41_HV201 NSWDVFGNWFDLTS GP41_HVZG1 NSWDVFGNWFDLTS GP41_HV2BE NSWDILGNWFDLTS GP41_HV2NZ NSWDVFTNWLDFTS GP41_HV2CA NNWDVFTNWFDLTS GP41_HV2RO NSWDIFGNWFDLTS GP41_HV2S2 NSWDVFSNWPDLTS GP41_HV2ST NSWDVFGWWFDLTS GP41_HV2SB NSWDVFCNWFDLTS GP41_HV2D2 NSWDVFGNWFDLTS *.**:: **:*:**

TABLE 2c GP36_FIVPE QYHQVLATHQEAIEKVTGALKINNLRLVTLEHQVLVIGLKVEAMEKFLYTAFAMQELGCN GP36_FIVU1 QYHQVLATQQEAILKVTEALKITNLRLVTLEHQVLVIGLKVEAMEKFLYTAFAMQELGCN GP36_FIVWO QYQQVLATHQEAIEKVTEALKINNLRLVTLEHQVLVIGLKVEAMEKFLYTAFAMQELGCN GP36_FIVU2 QYHQVLATHQETIEKITEALKVNNLRLVTLEHQVLVIGLKVEAIEKLFYTAFAMQELGCN GP36_FIVU8 QYHQVLATHQETIEKVTEALKINNLRLVTLEHQVLVIGLKVEAMEKFLYTAFAMQELGCN GP36_FIVSD QYQQVLATHQEALDKITEALKINNLRLVTLEHQMLVIGLKVEAIEKFLYTAFAMQELGCN GPJ6_FIVT2 QYHQVLATHQOALEKITEALKINNLRLITLEHQVLVIGLRVEAIEKFLYTAFAMQELGCN **:*****:*::::*:* ***:.****:*****:*****:***:**************** GP36_FIVPE QNQFFCKIPLELWTRYNMTINQTIWNHGNITLGEWYNQTKDLQQKFYEIIMDIEQNNVQG GP36_FIVU1 QNQFFCKVPPELWRRYNMTINQTIWNHGNITLGEWYNQTKDLQKKFYGIIMDIEQNNVQG GP36_FIVWO QNQFFCKVPSALWERYNMTINQTIWNHGNITLGEWYNQTKDLQQRFYEIIMDIEQNNVQG GP36_FIVU2 QNQFTCKVPPELWQRYNMTINQTIWNHGNITLGEWYNQTKDLQQKFYEIIMDMEQNNVQC CP36_FIVU8 QNQFFCKVPPELWKRYNMTINQTIWNHGNITLGEWYNQTKELQQKFYEIIMNIEQNNVQV GP36_FIVSD QNQFFCKIPKELWLRYNMTLNQTIWNHGNITLGEWYNQTKYLQQKFYEIIMDIEQNNVQG GP3E_FIVT2 QNQFFCKIPPSLWSMYNMTLNQTIWNHGNISLGNWYNQTRDLQNKFYEIIMDIEQNNVQG ******::* ** ****:**********:**:*****: **::** ***::****** GP36_FIVPE KTGIQQLQKWEDWVRWIGNIPQ GP36_FIVUl KKGLOQLOKWEDWVGWIGNIPQ GP36_FIVWO KKGLQQLQEWEDWVGWIGNIPQ GP36_FIVU2 RKGLQQLQEWEDWVCWLGNIPR GP36_FIVU9 KKGLQQLQEWEDWVGWIGNIPQ GP36_FIVSD KQCLQKLQNWQDWNGWIGKIPQ GP36_FIVT2 KTGIQQLQKWENWVGWIGKIPQ : *:*:**:*::*: *:*:**:

TABLE 2d GP41_SIVMK QSRTLLAGIVQQQQQLLGVVKRQQELLRLTVWGTKNLQTRVTAIEKYLEDQAQLNAWGCA GP41_SIVML QSRTLLAGIVQQQQQLLGVVKRQQELLRLTVWGTKNLQTRVTAIEKYLEDQAQLNAWGCA GP41_SIVM1 QSRTLLAGIVQQQQQLLGVVKRQQELLRLTVWGTKNLQTRVTAIEKYLEDQAQLNAWGCA GP41_SIVS4 QSRTLLAGIVQQQQQLLGVVKRQQELLRLTVWGTKNLQTRVTAIEKYLEDQAQLNAWGCA GP41_SIVSP QSRTLLAGIVQQQQQLLGVVKRQQELLRLTVWGTKNLQTRVTAIEKYLEDQAQLNAWGCA GP41_SIVAC QSRTLLAGIVQQQQQLLGVVKRQQELLRLTVWGTKNLQTRVTAIEKYLEDQAQLNAWGCA CP41_SIVAT QSRTLLAGIVQQQQQLLGVVKRQQELLRLTVWGTKNLQTRVTAIEKYLEDQAQLNAWGCA GP41_SIVA1 QSRTLLAGIVQQQQQLLGVVKRQQELLRLTVWGTKNLQTRVTAIEKYLEDQAQLNAWGCA GP41_SIVAI QSRTLLAGIVQQQQQLLGVVKRQQELLRLTVWGTKNLQTRVTAIEKYLEDQAQLNAWGCA GP41_SIVGB QSRTLLAGIVQQQQQLLGVVKRQQELLRLTVWGTKNLQTRVTAIEKYLEDQAQLNAWGCA GP41_SIVCZ QAROLLSGIVQOONNLLKAIEAOOHLLOLSIWGVKQLOARLLAVERYLQDQQILGLWGCS *:: *::**::**::**  :  *..:*:*::**.*:*:::: ::*.*: **  *. ***: GP41_SIVMK FRQVCHTTVPWPNASL-----TPDWNNDTWQEWERKVDFLEENITALLEEAQIQQEKNMY GP41_SIVNL FRQVCHTTVPWPNASL-----TPDWNNDTWQEWERKVDFLEENITALLEEAQIQQEKNMY GP41_SIVM1 FRQVCHTTVPWPNASL-----TPDWNNDTWQEWERKVDFLEENITALLEEAQIQQEKNMY GP41_SIVS4 FRQVCHTTVPWPNASL-----TPDWNNDTWQEWERKVDFLEENITALLEEAQIQQEKNMY GP41_SIVSP FRQVCHTTVPWPNASL-----TPDWNNDTWQEWERKVDFLEENITALLEEAQIQQEKNMY GP41_SIVAG WKQVCHTTVPWQWNNR-----TPDWNNMTWLEWERQISYLEGNITTQLEEARAQEEKNLD GP41_SIVAT WKQVCHTTVPWQWNNR-----TPDWNNMTWLEWERQISYLEGNITTQLEEARAQEEKNLD GP41_SIVA1 WKQVCHTTVPWQWNNR-----TPDWNNMTWLEWERQISYLEGNITTQLEEARAQEEKNLD GP41_SIVAI WKQVCHTTVPWQWNNR-----TPDWNNMTWLEWERQISYLEGNITTQLEEARAQEEKNLD GP41_SIVGB WKQVCHTTVPWQWNNR-----TPDWNNMTWLEWERQISYLEGNITTQLEEARAQEEKNLD GP41_SIVCZ GKAVCYTTVPWNNSWPGSNSTDDIWGNLTWQQWDKLVSNYTGKIFGLLEEAQSQQEKNER    **: :*               * . ** :*:        :*   * :*  ::.:* GP41_SIVMK ELQKLNSWDVFGNWFDLAS GP41_SIVML KLQKLNSWDVFGNWFDLAS GP41_SIVN1 ELQKLNSWDVFGNWFDLTS GP41_SIVS4 ELQKLNSWDIFGNWFDLTS GP41_SIVSP ELQKLNSWDIFGNWFDLTS GP41_SIVAG AYQKLSSWSDFWSWFDFSK GP41_SIVAT AYQKLTSWSDFWSWFDFSK GP41_SIVAl AYQKLSDWSSFWSWFDPSK GP41_SIVAI LYQKLDDWSGFWSWFSLST GP41_SIVGB ELQKLGDLTSWASWFDFTW GP41_SIVCZ DLLELDQWASLWNWFDITK     :* .     .**.::

TABLE 3 gp41 (region 545-682) 545-QARQLLSGIVOQQNNLLRAIEAOOHLLQLTVWGIK QLQARILAVERYLKDQQLLGIWGCSGKLICTTAVP WNASWSNKSLEQIWNNMTWMEWDREINNYTSLIHS LIEESQNQQEKNEQELLELDKWASLWNWFNITN-682

TABLE 3bis Regions of gp41 Region 555-577 QQQNNLLRAIFAQQHLLQLTVWG Region 572-601 QLTVWGIKQLQARILAVERYLKDQQLLGIW Region 590-620 RYLKDQQLLGIWGCSGKLICTTAVPWNASWS Region 628-663 WNNMTWMEWDREINNYTSLIHSLIEESQNQQEKNEQ

TABLE 4 IL-2 Region 27-47 TKKTQLQLEHLLLDLQMILNG Region 45-69 LNGINNYKNPKLTRMLTFKFYMPKK Region 99-121 HLRPRDLISNINVIVLELKGSET Region 131-153 TATIVEFLNRWITFCQSIISTLT

TABLE 4bis gp41 IL-2 Region 572-601 QLTVWGIKQLQARILAVERYLKDQQLLGIW Zone 27-47 TKKTQLQLEHLLLDLQMILNG

TABLE 5 Mutations at the level of region 555-577 Region 555-577 QQQNNLLRAIEAQQHLLQLTVWG Mutation 1: QQQNNLLAAIEAQQHLLQLTVWG Mutation 2: QQQNNLLRAIERQQHLLQLTVWG Mutation 3: QQQNNLLAAIERQQHLLQLTVWG Mutation 4: QQQNNLLRAIEAQOELLQLTVWG Mutation 5 QQQNNLLRAIEAQQQLLQLTVWG Mutation 6 QQQNNLLRTUEAQQHLLRLTVWG Mutation 7 QQQNNLLRAIEAQQHLLKLTVWG Mutation 8 QQQNNLLRAIEAQQQLLKLTVWG Mutations at the level of region 572-601 Region 572-601 QLTVWGIKQLQARILAVERYLKAQQLLGIW Mutation 1: QLTVWGIKQLQARILAVERYLKAQQLLGIW Mutation 2: QLTVWGIKQLQARILAVEAYLKDQQLLGIW Mutation 3: QLTVWGIKQLQARILAVEAYLKAQQLLGIW Mutation 4: QLTVWGIKQLQARILAVEDYLKRQQLLGIW Mutation 5: QLTVWGIKQLQARITAVERYLKDQQLLGIW Mutations at the level of region 590-620 Region 590-620 RYLKDQOLLGIWGCSGKLICTTAVPWNASWS Mutation 1: KYLKDQQLLGIWGCSGKLICTTAVPWNASWS Mutation 2: RYLKDQALLGIWGCSGKLICTTAVPWNASWS Mutation 3: RYLKDQQQLGIWGCSGKLICTTAVPWNASWS Mutation 4: RYLKDQAQLGIWGCSGKLICTTAVPWNASWS Mutation 5: RYLKDQARLGIWGCSGKLICTTAVPWNASWS Mutation 6: RYLKDQQLLNSWGCSGKLICTTAVPWNASWS Mutation 7: RYLKOQQLLGIWGCSQKLICTTAVPWNASWS Mutation 8: RYLKDQOLLGIWGCSFXLICTTAVPWNASWS Mutation 9: RYLKDQQLLGIWGCSGKLICTTAVPWNASSS Mutation 10: RYLKDQQLLGIWGCSGKLICTTAVPWNATL Mutation 11: RYLKDQQLLGIWGCSGKLICTTAVPWNTNR Mutation 12: RYLKDQQLLGIWGCSGKLICTTAVPWNTNR Mutation 13: RYLKDQQLLGTWGCSGKLICTTAVPWNANTS Mutations at the level of region 628-663 Region 628-663 WNNMTWMEWDREINNYTSLIHSLIEESQNQQEKNEQ Mutation 1: WNNMTWMEWDRETNNYESLIHSLIEESQNQQEKMEQ Mutation 2: WNNMTWMEWDRETNNYTSNIHSLIEESQNQQEKMEQ

TABLE 6 Region 651-673 EAIEKVTGALKINNLRLVTLEHQ Mutation 1 EAIEKVT R ALKINNLRLVTLEHQ Mutation 2 EAIEKVT D ALKINNLRLVTLEHQ Mutation 3 EAIEKVTR A LKINNLRLVTLEHQ Mutation 4 EAIEKVTQ A LKINNLRLVTLEHQ Region 668-697 VTLEHQVLVIGLKVEAMEKFLYTAFAMQEL Mutation 1 VTLEHQVLVIGLKVFAME A FLYTAFANQEL Mutation 2 VTLEHQVLVIGLKVEAME N FLYTAFANQEL Mutation 3 VTLEHQVLVIGLKVEANE Y FLYTAFAMQEL Mutation 4 VTLEHQVLVIGLKVEAME R FLYTAFAMQEL Mutation 5 VTLEHQVLVIGLKVEAMEKFL K TAFAMQEL Mutation 6 VTLEHQVLVIGLKVEAMEKFL E TAFAMQEL Mutation 7 VTLEHQVLVIGLKVEAMEKFE Q TAFAMQEL Mutation 8 VTLEHQVLVIGLKVEAMEKFL R TAFANQEL Mutation 9 VTLEHQVLVIGLKVEAMEKFL A TAFAMQEL Mutation 10 VTLEHQVLVIGLKVEAMEICFLYTAF Q MQEL Mutation 11 VTLEHQVLVIGLKVEAMEKFLYTAF K MQEL Mutation 12 VTLEHQVLVIGLKVEAMEKFLYTAF R MQEL Mutation 13 VTLEHQVLVIGLKVEANEKFLYTAFAMQIL Mutation 14 VTLEHQVLVIGLKVEAMEKFLYTAFAMQ A L Mutation 15 VTLEHQVLVIGLKVEANEKFLYTAFAMQ F L Mutation 16 VTLEMQVLVIGLKVEAMEKFLYTAFAMQ F L Region 686-718 KFLYTAFAMQELGCNQNQFFCKIPLELWTRYNM Mutation 1 KFLYTAFAMQELGCNQN K FFCKIPLELWTRYNM Mutation 2 KFLYTAFAMQELGCNQN R FFCKIPLELWTRYNM Mutation 3 KFLYTAFAMQELGCNQN G FFCKIPLELWTRYNM Mutation 4 KFLYTAFANQELGCNQN A FFCKIPLELWTRYNM Mutation 5 KFLYTAFAMQELGCNQNQ L FCKIPLELWTRYNM Mutation 6 KFLYTAFANQELGCNQNQ H FCKIPLELWTRYNM Mutation 7 KFLYTAFAMQELGCNQN Q IFCKIPLELWTRYNM Mutation 8 KFLYTAFAMQELGCNQNQ A FCKIPLELWTRYNM Mutation 9 KFLYTAFANQELGCNQNQ Q FCKIPLELWTRYNM Mutation 10 KFLYTAFANQELGCNQNQ RFCKIPLELWTRYNM Region 727-762 WNHGNITLGEWYNQTKDLQNKFYEIIMDIEQNNVQGKT Mutation 1 WNHGNITLGEWYNQTKDLQ N KFYEIIMDIEQNNVQGKT Mutation 2 WNHGNITLGEWYNQTKDLQ H QKFYEIIMDIEQNNVQGKT Mutation 3 WNHGNITLGEWYNQTKDLQ S KFYEKFMDIEQNNVQGKT Mutation 4 WNHGNITLGEWYNQTKDLQ A KFYEIIMDIEQNNVQGKT Mutation 5 WNHGNITLGEWYNQTKDLQ G KFYEIIMDIEQNNVQGKT Mutation 6 WNHGNITLGEWYNQTKDLQ E KFYEIIMDIEQNNVQGKT 

1. A method of searching for and obtaining a vaccine against the pathogenic effects related to the infection of an animal or human host by a retrovirus capable of penetrating into a target cell of said host, said target cell possessing a membrane receptor for a protein of said host, comprising: a) preparing candidate vaccine agents based on a polypeptide comprising at least part of an envelope protein of a pathogenic strain of said retrovirus, said polypeptide being present, in said vaccine agents, in a modified form, wherein: said part of the envelope protein is chosen from those which comprise at least one fragment of an immunodominant region of said envelope protein, said fragment containing at least one amino acid which is a conserved amino acid of said immunodominant region and which is present in said pathogenic strain, said polypeptide is chosen from those which, in the unmodified state, induce an immune response directed both against said immunodominant region and against the protein of the host, and b) selecting as vaccine such a modified polypeptide chosen from those which induce an immune response directed against said immunodominant region of the envelope protein and not against the protein of the host.
 2. The method of claim 1, wherein said immunodominant region is chosen: from those which give a cross-reaction, of the B type and/or of the T type, with said protein of the host, and/or from those for which a three-dimensional structural analogy with part of said protein of the host has been determined beforehand.
 3. The method of claim 1, wherein: said protein of the host is a soluble mediator; and/or said soluble mediator is chosen from growth factors, protein hormones and cytokines; and/or said cytokine is interleukin-2, interleukin-10, interleukin-15, interleukin-8 or a chemokine.
 4. The method of claim 1, wherein: said virus is chosen from the following: HIV, FIV, SIV, ALV, MULV, FELV, HTLV, STLV, BLV, Rous sarcoma virus, Maedi-visna virus, feline sarcoma virus, avian myelocytomatosis virus, avian myeloblastosis virus, primate type D retroviruses, mammary tumor-inducing type B retroviruses; said envelope protein is a transmembrane glycoprotein of said retrovirus; said transmembrane glycoprotein is that of an HIV, FIV or SIV virus; said glycoprotein is the gp41 protein of HIV1; said vaccine agent contains, in modified form, a polypeptide comprising at least part of region 545-682 of the gp41 protein of HIVI; said glycoprotein is the gp36 protein of FIV.
 5. The method of claim 1, wherein said vaccine agent is an oligomer of at least part of a transmembrane glycoprotein of said retrovirus in modified form.
 6. The method of claim 1, wherein said modified polypeptides are obtained by the production of mimotopes.
 7. The method of claim 1, wherein said retrovirus is HIV.
 8. The method of claim 1, wherein said retrovirus is FIV.
 9. A vaccine obtainable by the method of claim
 1. 10. A vaccine against the pathogenic effects related to the infection of an animal or human host by a retrovirus, said retrovirus being capable of penetrating a target cell of said host possessing a membrane receptor for a protein of said host, said retrovirus possessing an envelope protein inducing an immune response directed both against a conserved and immunodominant region of said envelope protein and against said protein of the host, wherein said vaccine contains a vaccine agent capable of inducing an immune response directed against said region of said envelope protein and not against said protein.
 11. The vaccine of claim 10, wherein said retrovirus is HIV.
 12. The vaccine of claim 10, wherein said retrovirus is FIV.
 13. The vaccine of claim 10, wherein said vaccine agent is a mimotope of at least part of said conserved and immunodominant region.
 14. An antibody obtainable by immunizing a host with the aid of a vaccine according to claim 10, wherein said antibody recognizes said envelope protein and not the protein of the host.
 15. A pharmaceutical composition containing an antibody of claim
 14. 16. A method for preventing the pathogenic effects related to the infection of an animal or human host by a retrovirus capable of penetrating into a target cell of said host, said target cell possessing a membrane receptor for a protein of the host, and said retrovirus possessing an envelope protein inducing an immune response directed both against a conserved and immunodominant region of said envelope protein and against said protein of the host, wherein said method comprises administering to said host a modified polypeptide as defined in claim
 1. 17. A method for preventing the pathogenic effects related to the infection of an animal or human host by a retrovirus capable of penetrating into a target cell of said host, said target cell possessing a membrane receptor for a protein of the host, and said retrovirus possessing an envelope protein inducing an immune response directed both against a conserved and immunodominant region of said envelope protein and against said protein of the host, wherein said method comprises administering to said host a vaccine of claim
 9. 18. A method for preventing the pathogenic effects related to the infection of an animal or human host by a retrovirus capable of penetrating into a target cell of said host, said target cell possessing a membrane receptor for a protein of the host, and said retrovirus possessing an envelope protein inducing an immune response directed both against a conserved and immunodominant region of said envelope protein and against said protein of the host, wherein said method comprises administering to said host a vaccine of claim
 10. 19. A polynucleotide encoding a modified polypeptide as defined in claim
 1. 20. Expression vector containing the polynucleotide of claim
 19. 